The present invention is concerned with explosive shaped-charges, and more particularly to an improved liner for use in such shaped-charges, an improved shape charge which is especially useful in a well pipe perforating gun, and a method for making them.
The use of shaped-charges for perforating the tubing, pipes, or casings used to line wells such as oil and natural gas wells and the like, is well-known in the art. For example, U.S. Pat. No. 3,128,701, issued Apr. 14, 1964 to J. S. Rinehart et al, discloses a shaped-charge perforating apparatus for perforating oil well casings and well bore holes.
The art has also devoted attention to providing a particular configuration of the shaped-charge and its liner as shown, for example, in U.S. Pat. No. 5,221,808, issued Jun. 22, 1993 to A. T. Werner et al. The shaped-charge therein disclosed includes the usual case, concave shaped explosive material packed against the inner wall of the case, and a metal liner lining the concave side of the shaped explosive. As disclosed in the paragraph bridging columns 3 and 4 of the patent, the taper is said to exist in the thickness of the liner 14 starting at the apex 18 thereof and ending with the skirt 16 thereof. At the first ten lines of column 4, specifications are given for the copper-bismuth liner 14 including a maximum variation in thickness along any given transverse section of the liner, a specified thickness of the skirt 16 of the liner 14, and the taper of the liner at the apex 18 and the skirt 16. U.S. Pat. No. 5,509,356 issued Apr. 23, 1996 to Steven L. Renfro, the disclosure of which is incorporated herein by reference, also addresses control of liner thickness. The disclosure of this patent proposes a spinning manufacturing process to produce a liner having a closed end apex 5% to 50% thicker, preferably 25% thicker, than its skirt.
Generally, shaped-charges utilized as well perforating charges include a generally cylindrical or cup-shaped housing having an open end and within which is mounted a shaped explosive which is configured generally as a hollow cone having its concave side facing the open end of the housing. The concave surface of the explosive is lined with a thin metal liner which, as is well-known in the art, is explosively driven to hydrodynamically form a jet of material with fluid-like properties upon detonation of the explosive and this jet of viscous material exhibits a good penetrating power to pierce the well pipe, its concrete liner and the surrounding earth formation. Typically, the shaped-charges are configured so that the liners along the concave surfaces thereof define simple conical liners with a small radius apex at a radius angle of from about 55 degrees to about 60 degrees. Other charges have a hemispherical apex fitted with a liner of uniform thickness.
Generally, explosive materials such as HMX, RDX, PYX, or HNS are coated or blended with binders such as wax or synthetic polymeric reactive binders such as that sold under the trademark KEL-F. The resultant mixture is cold- or hot-pressed to approximately 90% of its theoretical maximum density directly into the shaped-charge case. The resulting shaped-charges are initiated by means of a booster or priming charge positioned at or near the apex of the shaped-charge and located so that a detonating fuse, detonating cord or electrical detonator may be positioned in close proximity to the priming charge.
The known prior art shaped-charges are typically designed as either deep-penetrating charges or large-diameter hole charges. Generally, shaped-charges designed for use in perforating guns contain 5 to 60 grams of high explosive and those designed as deep-penetrating charges will typically penetrate concrete from 10 inches to over 50 inches. Large-diameter hole shaped-charges for perforating guns create holes on the order of about one inch in diameter and display concrete penetration of up to about 9 inches. Such data have been established using API RP43, Section I test methods.
The embodiments of the present invention involve a shaped-charge liner, a shaped-charge explosive incorporating the liner, and methods for making the liner. The liner of the present invention includes a convex outer surface, a concave inner surface, an apex having a center, and a mouth portion of the liner opposite the apex of the liner. The liner also incorporates a skirt portion terminating in a circular skirt edge at the mouth portion of the liner. In the preferred embodiment of the liner, at least some of the skirt portion of the liner has had material removed by machining reducing the thickness of the skirt portion and as a result, the machined skirt portion has a thickness within about 25% of the thickness of the material around the center of the apex. Additionally, the liner may incorporate a circular opening at the center of the apex where the ratio of the diameter of the opening to the diameter of the circular skirt edge is between about 0.05 and about 0.35.
In an alternative embodiment of the liner, at least some of the skirt portion of the liner has been machined to a rough machine finish, but without necessarily removing significant amounts of material. In this alternative embodiment, the mass of the material removed in the machining is less than 5% of the mass of the liner, more preferably less than 1% of the mass of the liner, and most preferably less than 0.1% of the mass of the liner.
The liner of the present invention may be incorporated into a shaped-charge. Such a shaped-charge would include a housing having an inner wall, an outer wall, a base, and a mouth portion opposite the base, a shaped-explosive having an open concave side and mounted on the inner wall of the housing with the concave side of the shaped explosive facing the mouth portion of the housing, and the liner, preferably having an opening at the center of the apex. The liner would line the concave side of the shaped explosive, leaving an open space between the liner and the mouth portion of the housing. The preferred embodiment of the shaped-charge would also include a coating at the opening at the center of the apex of the liner; where the coating contacts the shaped-explosive and the open space between the liner and the mouth portion of the housing. This coating could be single or multiple layers, but would preferably include an adhesive.
The liner of the present invention could be made by more than one method. The preferred method would involve drawing a flat material into a concave shape radially symmetric about a central axis having an apex centered on the central axis and a mouth at the opposite end from the apex. In this act, the center of the material is drawn down to form the apex while the perimeter of the material forms a skirt portion terminating in a circular skirt edge at the mouth of the liner. The method would also call for removing any excess flat material outside the circular skirt edge forming the mouth. Finally, the method would also include machining at least some of the skirt portion removing material and thereby reducing the thickness of the skirt portion.
One alternative method for making the liner would use a spinning process rather than a drawing process. This method would include spinning a sheet of material into a concave shape radially symmetric about a central axis having an apex centered on the central axis and a mouth at the opposite end from the apex, wherein a portion of the material forms the apex and a portion of the material forms a skirt portion terminating in a circular skirt edge at the mouth of the liner. The method would again involve removing any excess material outside the circular skirt edge forming the mouth and machining at least some of the skirt portion removing material and thereby reducing the thickness of the skirt portion. This method could also include machining the apex of the liner removing material and thereby reducing the thickness of the apex until the thickness of the apex is within about 25% of the thickness of the skirt portion.